Cadillac DeVille 1985 to 2005 including:1985-1992 Fleetwood, 1993 Sixty Special, All FWD Forum Discussion, Pinging in Past Cadillac Vehicle Discussion; Sounds like you found the vacuum reservoir.
As for the Top-Engine Cleaner, if you did it right, you can rule ...

Sounds like you found the vacuum reservoir.
As for the Top-Engine Cleaner, if you did it right, you can rule out carbon build-up as the cause of the "pinging". The timing chain is possible but you should also look into anything that could be causing a "lean" condition, and make sure your EGR system is functional.

Could be possible that none of that stuff was ever checked..... It would be something that needs to be done, but may not change the ping.... I just replaced the air filter FOR THE FIRST TIME today on the fleetwood, it had NEVER been changed after 36k!!! It was black and icky.. Thats what they get for letting a private shop take care of all the maintenance....

shoot . did the o2 sensors , i no longer get the intermiottent "service engine soon light" now i get a "service vehicle soon " message on the scrolling display in addition to the "service suspension system" message

im about give up at this point , im keeping the radio way up and trying find a way to turn off all warnings and drive it , im trying my best to maintin it but im fighting what appears to be a looseing battle..i miss my infiniti